This invention relates to opto-electronic devices and, more particularly, to a photodetector for converting an optical signal to an electrical signal. Specifically, one embodiment of the invention is directed to a p-i-n photodiode which has a uniform detection characteristic in response to radiation that is distributed across the surface of the photodiode and which can be structured to operate over a broad range of optical wavelengths, such as from 0.4 to 1.6 .mu.m. For example, such a p-i-n photodiode has potential use in photonics instrumentation, such as an optical spectrum analyzer.
InP/InGaAs/InP p-i-n photodiodes and avalanche photodiodes are the typical candidates for optical-fiber communications in the 1.0 to 1.6 .mu.m wavelength range. See, for example, Carey, K. W., Wang, S. Y., Hull, R., Turner, J. E., Oertel, D., Bauer, R., and Bimberg, D., "Characterization of InP/InGaAs/InP Heterostructures Grown by Organometallic Vapor Phase Epitaxy For High-Speed p-i-n Photodiodes," Journal of Crystal Growth, 77, 558-563, (1986), and Moseley, A. J., Urquhart, J., Hodson, P. D., Riffat, J. R., and Davies, J. I., "InGaAs/InP SAGM Avalanche Photodiodes Incorporating a Pseudoquaternary Superlattice Graded Heterojunction Grown by Atmospheric-Pressure MOCVD," Electronics Letters, 23, 914-915, (1987). These p-i-n photodiodes are the present photodetectors-of-choice because of their superior bandwidth. Bowers, J. E., and Burrus, C. A., "InGaAs PIN Photodetectors with Modulation Response To Millimeter Wavelengths," Electronics Letters, 21, 812-814, (1985), and Tucker, R. S., Taylor, A. J., Burrus, C. A., Eisenstein, G., Wiesenfeld, J. M., "Coaxially Mounted 67 GHz Bandwidth InGaAs PIN Photodiode," Electronics Letters, 22, 917-918, (1986). Also, these p-i-n photodiodes are considered superior because of their low dark current characteristic. Ohnaka, K., Kubo, M., and Shibata, J., "A Low Dark Current InGaAs/InP p-i-n Photodiode With Covered Mesa Structure," IEEE Transactions On Electron Devices, ED-34, 199-204, (1987).
A window layer, that is, the InP top layer in an InP/InGaAs/InP p-i-n photodiode structure, which is typically an InP:Zn p.sup.+ material, is needed in the structure of these photodiodes in order for them to provide a fast response. If an InGaAs p.sup.+ layer were substituted for the InP:Zn p.sup.+ top layer to provide an InGaAs p.sup.+ /InGaAs n.sup.- /InP:S structure, the resulting p-i-n photodiodes would need to rely on the diffusion characteristic of the InGaAs n.sup.- material to operate, which would result in an unacceptably slow response. However, the resistance of the InP:Zn p+ layer is not low enough to assure uniform response of the p-i-n photodiode when radiation is distributed across the surface of the entire photodiode. Considered in more detail, the known InP/InGaAs/InP p-i-n photodiode structure is shown in FIG. 1. This p-i-n photodiode typically comprises an InP:Zn/InGaAs/InP:S structure 1,2,3 disposed between top Ti/Au/AuZn/Au and bottom Au-Ge/Ni/Au contacts 4, 5, respectively. As shown in FIG. 1, a buffer layer 6, for example, InP, can be incorporated between the InGaAs n.sup.- and InP:S layers 2 and 3, respectively. The InP:Zn p+ layer is typically on the order of 1.0 to 1.1 .mu.m thick. Unfortunately, the InP:Zn p.sup.+ layer 1 is thick enough that a relatively long growing time is needed. This causes diffusion of Zn into the InGaAs n.sup.- layer 2. Consequently, there is a migration of p dopant from the InP:Zn p+ layer 1 into the InGaAs n.sup.- layer 2, that is, the Zn dopes the InGaAs material p.sup.+. This means that the p-i-n photodiode begins to act as though the InP:Zn p.sup.+ window layer 1 is eliminated. Accordingly, carriers move more slowly in the InGaAs n.sup.- layer 2, which is evidenced by the p-i-n photodiode having a slow response.
A single, large area p-i-n photodiode having a fast response could potentially be used to detect light from both single mode and multi-mode optical fibers, which produce spots of different diameters. However, the need to detect light beams of different diameters also mandates not only a fast response, as provided by known p-i-n photodiodes, but also a uniform photodetector response characteristic across the detecting area. Unfortunately, these known InP/InGaAs/InP p-i-n photodiodes do not provide a uniform response.
It is therefore desirable to provide a p-i-n photodiode having a fast response, which also has a uniform photodetector response characteristic across the detecting area. It is also desirable that the p-i-n photodiode operate over a broad range of optical wavelengths.